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Use your soil nutrient bank

Editor’s note: This article is from the archives of the MSUCrop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included

With
the significant increases in costs of phosphate and potash fertilizers
since last season and with tight supplies, some reflection on
recommended rates is in order. MSU’s recommendations for phosphate
(P2O5) and potash (K2O) are based on the buildup and maintenance
approach. In this approach, the most important soil test value is the
critical level (CL). On average, when the soil test value is at the CL,
there is sufficient P or K available in the soil to produce top quality
produce and near maximum yield. This is usually very close to the point
of maximum economic return. For P, the CL varies by crop. For most
vegetable crops, it is 40 ppm. For K, the CV varies with the cation
exchange capacity of the soil. At or above the CL applying more
fertilizer will usually not improve yield, but if no P or K is added,
the available level in the soil will decline.

An
adequate soil test or maintenance range has been established over which
an amount of P or K equal to crop removal is recommend. For phosphorus,
this range is 25 to 30 ppm beyond the CV and for K it is 20 ppm. When
the soil test value is beyond the maintenance range, there is no need to
apply addition P or K. For most vegetable crops, no P is necessary when
the soil test value is over 70 ppm. With K, the soil test value above
which no K is needed will range from 105 to 140 ppm depending on soil
texture, sandy to clayey. When the P or K soil test value is below the
CL, the recommended amount of that nutrient will include an amount to
build up the soil test value to the CL over a four year period plus an
amount equal to crop removal (maintenance). Crop removal information is
available in MSUE bulletin Nutrient Recommendations for Vegetable Crops in MichiganE-2934,
page 9. Due to higher prices and limited supplies, farmers should
consider applying no more than crop removal. This will provide an
adequate amount for top yields and maintain the current available P or K
level in the soil.

Many
Michigan soils used for vegetable crop production have more than
adequate amounts of P. In those situations, no or only a small amount of
phosphorus in the planting time fertilizer is needed. Make use of the P
in your soil bank. There is a greater potential for benefit from
applying potassium (K) than for P. Only 25 to 30 percent of vegetable
crop soils contain more than adequate available K. With limited supplies
of potash this year decisions may need to be made on how to allocate
the potash you can get. Including 20 to 25 lbs K2O in planting time
fertilizer can give improved early growth and K use efficiency and allow
a reduction in the amount being broadcast. Due to the nature of the
yield response to applied K, it is best to apply a reduced amount (e.g.
75 to 80 percent of recommended amount) to all fields needing K rather
than applying the full amount to some fields and a small amount to other
fields. Soil test results provide a good guide for how best to allocate
available fertilizer resources. Those that have soil test information
from the past year or two are in a good position to fertilizer
allocation decisions. There is still time before planting to collect
soil samples and get the soil test information for fields not recently
tested.

November 14, 2016 | Jim Isleib | If soil calcium and magnesium levels are adequate and soil pH is acceptable, variations in the calcium-to-magnesium ratio between 2 and 8 have been shown to have no influence on crop yield.